A person who posts things on some online community setting that are designed to purposefully assault and offend large numbers of people for no reason other than the Troll's bias against them, hatred for them or personal amusement at annoying them.

Guy 1: Have you been on Encyclopaedia Dramatica?
Guy 2: No!! That place is for Trolls!!
Troll: OMG JOO PEDOPHILE!! U R TEH GAY 1D10T!! G0 DIE JOO FAIL!! LULZ IS GUD ROFLMAO!!
Guy 1: Ah. I see what you mean.

A person on the internet (usually YouTube) who doesn't have anything to do with their life so they go around starting arguments.

There are:
Plain trolls- Average troll
Bashing trolls- Go around yelling and swearing and not using periods.
Smartass trolls- Use big words to try and sound smart
Non caring trolls- Claim to not care even though they do
Opinion trolls- Are prejudice
12 year old trolls- Can't spell one word right or punctuate
Blaming troll: Looks for reasons to hate someone.

Plain troll:
"How dumb do you have to be to like this? My god, get off the internet"

Smartass troll:
"So she stopped singing in the middle of her concert to sign a painting? But how inconsiderate of her! That shows she has no affection for her audience. How did you know they wanted that painting signed? She ruined it, she bastardized the beauty of something someone worked so hard on. It clearly shows how everyone's maturity towers above hers."

Non caring troll:
"Yeah, I didn't ask for this. I don't care if you're overreacting because I posted a comment. Stop wasting your time and don't reply."

12 year old troll:
"dis rly suks i men srsly how can ay1 evr lyke dis lol"

Blaming troll:
"But when you pause at 0:32 you can see a triangle. And when you play the video backwards, you can hear "the devil I love". This person is clearly satanic and trying to brainwash you."

Opinion troll:
"Why are you getting all mad? Yeah, I said you're an idiot for liking this, I said this was the biggest waste of my time EVER, but I'm just stating my opinion, and you need to respect that."

somebody who posts on an internet forum or chatroom with the sole intention to upset, offend or cause argument. Often aimed towards a chatroom/forum moderator who has banned or timed them out, or other user whom they disagree with.